1. Field of the Invention
The present fnvention relates to a material handling car and track assembly and is directed more particularly to acceleration and deceleration means mounted in the track assembly and adapted to influence speed of the car.
2. Description of Prior Art
The prior ar has recognized the need for means to propel a car along a track without the installation of a driver mechanism or braking mechanism on the car. Systems have incorporated linear induction motors as an efficient means of achieving this. Such systems commonly employ single motors along a track whereby a car is propelled by a force created by the change in magnetic flux as the car passes by the motors. However, a problem inherent in these systems is that of undue stress on the cars and tracks themselves due to an electromagnetic force created between the car and the motor, causing the car to be attracted toward the track. This attractive force exerted on the car dissipates after the car passes away from the motor. In systems where there are commonly several hundred motors in use through which a car passes as it travels through the system, the constant attractive stress and release of such stress, leads to wear and tear on the car. As a result, cars break down and are in need of frequent repair.
Such a system is disclosed in U.S. Pat. No. 4,919,054 to Matsuo. In this system, linear induction motors are disposed in single file underneath the transport path of the car, causing the motors to exert an attractive force while acting to drive the car. Due to numerous motors in the system, a car undergoing such repeated application and release of stress becomes structurally weak and requires frequent servicing.
Another problem recognized in the prior art relates to providing a means for controlling the speed of a car through the use of a decelerating mechanism external of the car. Many systems which have employed inductive motors to impel a car forward along a track have relied entirely upon the same motors for braking the car. A reverse thrust is applied to the car by passing reverse phase alternating current through the coils of the motor stators, to slow the car down. However, these systems potentially overwork the motors which often leads to motor failure.
An example of this type of system is disclosed in U.S. Pat. No. 4,848,242 to Matsuo. As in the Matsuo '054 system, single motors are disposed at predetermined intervals underneath the transport path and, as in the previous system, an attractive force on the car is exerted and released as the car moves in the direction of the transport path. The motors impel the car in the forward and reverse directions. Thus, the motors carry out the function of starting and stopping the car, depending on the direction of current flow. However, this dual function leads to increased wear and tear on the motors.
In other systems, such as disclosed in U.S. Pat. No. 5,018,928 to Hartlepp, a car is decelerated through the use of a magnetic piston which moves by compressed air in a tube, such that a car will follow the piston. However, the use of such a structure is complicated and thus provides great opportunity for malfunction. Additionally, this system discloses the use of linear induction motors for propelling a car on a track wherein induction motors are singly disposed. Accordingly, an attractive force is exerted on the cars driven by the motor.
An additional problem recognized in the art is the need to mitigate the effects of motor failure. In U.S. Pat. No. 4,613,805 to Matsuo, there is disclosed a system providing for continued operation through the use of an auxiliary power source in the event that the main power source becomes disabled. However, this system does not insure continued operation in the event that a motor along the transport path should become disabled.